1. Field of the Invention
This invention is directed to an infrared intrusion detector, and more particularly to a passive infrared intrusion detector which provides a plurality of fields of view and detects the inherent infrared radiation emanating from a person passing through any of said fields of view.
2. Prior Art
There have been provided a wide variety of infrared detectors for monitoring the presence of a person or intruder entering in a room or space under surveillance to produce a signal representative of the detection. The most common type of such detector comprises at least one infrared sensing element and a plurality of concave reflector segments for providing separate fields of view to be monitored in the room or space. The concave reflector segments are arranged such that they collect and focus infrared radiation from the respective fields of view upon the sensing element for monitoring the presence of the person. To this end, the sensing element is designed to be located forwardly of the concave reflector segments for gathering the rays of infrared radiation once reflected by the reflector segments. In such a reflection system, the sensing element should be spaced forwardly of each reflection segment at a distance corresponding to each focal length from the corresponding reflector segment. Accordingly, the depth or thickness of a housing accommodating the sensing element and reflector segments must be large enough for covering such focal length, thus the depth or thickness of the housing cannot be reduced to beyond a certain limit. This has been the cause of a hindrance to obtain a more compact construction, particularly with respect to the thickness of the detector, which is highly desirable to be adapted in a restricted mounting space as well as which is desirable to be inconspicuous. With this arrangement, there also appears a problem that a wiring is required for connecting the sensing element in front of the reflector segments and electric components which are normally mounted on the backside of the detector and are cooperative with the sensing element to produce a cautionary signal representative of the infrared detection, such wiring detracting certainly from the neat arrangement of the detector and reducing design flexibility. In addition to the above, it is also desirable for such detectors having multi-spots detecting performance to kill the operation of one or more viewing fields in accordance with the actual condition of the room or space to be monitored by the detector. That is, problems are frequently seen in the case where one or more fields of view are occupied by such objects other than the human object that may emit infrared reradiation by being heated as by sunlight or other heat sources so as to mislead the detector, and in the case where one or more fields of view extend into unwanted regions which will only see passersby who have no intension of entering the space under surveillance. This occurs mostly when the detector is mounted for monitoring incoming or outgoing persons from the space such as a house or shop for the purpose of acknowledging the entrance into or exit from a supervised space. The prior art detectors, however, have been found not to have an easy access to selectively killing one or more fields of view depending upon the requirements of the room or space, and therefore are not to be satisfactory for the use in differing conditions.